A negative electrode for nonaqueous electrolyte secondary batteries is generally produced by mixing particles of an active material formed from a material into which lithium ions can be inserted by charging, with a binder, a conductive material and a solvent, applying the mixture thus obtained on the surface of a current collector, drying the mixture to form a coating film, and further subjecting the coating film to press processing.
In recent years, along with the development in applications such as electric vehicles and smart phones, there is an increasing demand for capacity increase and lengthening of the service life of batteries. Currently, most of the negative electrodes of commercially available batteries use graphite as the negative electrode active material; however, this active material has already reached the theoretical limit in terms of capacity, and it is now necessary to develop new negative electrode active materials. One of the promising candidates thereof is active materials containing silicon (also referred to as “silicon-based active materials”).
Silicon-based active materials have a potential that the capacity per mass is 5 to 10 times that of graphite. However, on the other hand, silicon-based active materials have a problem that electron conductivity is not so high compared with graphite. Thus, it has been hitherto suggested, in order to increase the electron conductivity of silicon-based active materials, to add a conductive auxiliary agent for the purpose of imparting electron conductivity between, for example, a current collector and the active material.
For example, it has been proposed in Patent Document 1 to attach particles of a metal material having a particle size of 0.0005 μm to 10 μm to the surfaces of silicon-based active material particles.
Furthermore, it has been proposed in Patent Document 2 to coat the periphery of core particles containing silicon with a silicon solid solution such as Mg2Si, CoSi or NiSi, and to further coat the surface with a conductive material such as graphite or acetylene black.
Also, since silicon-based active materials undergo large volumetric changes caused by insertion and desorption of lithium ions, silicon-based active materials also have a problem that detachment from the active material layer is likely to occur as charging and discharging are repeated, consequently deterioration of cycles or reduction of energy density occurs, the battery performance is decreased, and safety of the battery is decreased.
As a means for solving this problem, the applicant of the present invention previously suggested that an active material layer containing particles of an active material is provided, a metal material having a low ability to form lithium compounds is precipitated between the particles by electroplating, and the surface of the active material layer is coated continuously or non-continuously by a surface layer formed from a metal material of the same kind as the aforementioned metal material (Patent Document 3).
Furthermore, in regard to the silicon-based active materials, suggestions have been made to the effect of enhancing the battery characteristics by controlling the particle size distribution or the particle size.
For example, Patent Document 4 is described, in connection with active material particles containing silicon and/or a silicon alloy, to the effect that when the average particle size of the active material particles is adjusted to from 1 μm to 10 μm, and the particle size distribution is adjusted to a particle size distribution in which 60% by volume or more of the particles have a particle size in the range of from 1 μm to 10 μm, the volume of the active material particles expands and contracts along with the storage and release of lithium resulting from charge and discharge, and thereby an increase in the contact resistance between the active material particles is suppressed.
Patent Document 5 discloses, in connection with a negative electrode active material containing silicon particles, that the active material particles have an average particle size in the range of 7.5 μm to 15 μm, and have a particle size distribution in which 60% by volume or more of the particles have a particle size in the range of average particle size ±40%. It is disclosed to the effect that when the average particle size of the active material particles is adjusted to 7.5 μm or more, the number of particles per volume that exist in the thickness direction of the active material layer becomes smaller, and therefore, the number of particles that should be brought into contact with each other in order to obtain current collectability becomes smaller, so that satisfactory current collectability can be obtained.
Patent Document 6 discloses active material particles containing silicon, which have an average particle size of from 5 μm to 25 μm. When the average particle size of the active material particles is adjusted to 5 μm or more, the original specific surface area of the active material can be reduced. It is described to the effect that since the contact area between the electrolyte and the newly generated surfaces of the active material can be reduced thereby, the effect of enhancing the cycle characteristics and the effect of suppressing swelling of the active material are increased.